Aramid and PBI fibers are highly resistant to heat decomposition, have inherent flame resistant properties and are frequently used in working wear for special environments where flame resistant properties are required. Fabrics made of these fibers are extremely strong and durable, and have been widely adopted for use in the protective clothing field, particularly for military applications where personnel have the potential to be exposed to fire and flame, such as aircraft pilots, tank crews and the like. Various end uses for protective textile materials have different performance requirements. Fibers that are inherently flame resistant such as Nemox, PBI and Kevlar each have their own flame-resistant performance properties and are targeted for specific markets and applications. The process of this invention provides a means to increase the already excellent flame resistance values of such fibers and to "upgrade" certain fibers so that they may be used in applications for which they would not be otherwise qualified absent such treatment. In addition, enhanced flame resistant properties for fibers that are inherently flame resistant is always desirable.
There is a need for aramid, PBI and aramid/PBI blended fabrics that have flame resistant properties greater than that of the original fabric, dyed or undyed. As used herein, the term aramid fiber or fibers refers to meta-linked aromatic polyamide fibers made from high molecular weight polymers that are highly crystalline and have either a high or no glass transition temperature. These inherent desirable properties of aramid fibers also create difficulties for fiber processing in other areas; specifically, it is difficult to increase the inherent flame resistant properties of aramids to any significant extent with a durable flame retardant that withstands repeated launderings.
Aramid fibers suitable for the process of this invention include both dyed and undyed fibers. Dyed fibers from any source are successfully flame-retardant-treated, including those dyed using a polar organic solvent as a fiber swelling agent to introduce the dye into the fiber while in the swollen state. Procedures for dyeing and aramid fibers so produced are described in detail in copending, commonly assigned application Ser. No. 863,038 filed May 14, 1986, the disclosure of which is incorporated by reference.
Another source of dyed aramid fiber is the solution-dyed product, which is available from the fiber producer. In solution dyeing, a quantity of dye or pigment is mixed with the resin solution prior to extrusion of the resin into fine fibers; the dye or pigment becomes part of the fiber structure.
A further source of dyed aramid fibers results from the process described in U.S. Pat. No. 4,525,168 in which acid or anionic dyes are introduced into aramid fibers by coupling the dye to a dye site receptor which, in turn, is attached to the fiber.
It is an object of the present invention to provide a continuous process for improving the flame resistance of aromatic polyamide fibers, and blends of aramids such as Nomex with PBI either dyed or undyed, that will yield fibers having flame resistance greater than untreated fibers without detracting from the strength properties of the aramid fibers. Another object of this invention is to provide a continuous process adapted to enhance the already high flame resistance of solution dyed aramid fibers.